The inventive concepts described herein relate to semiconductor memories, and more particularly, to nonvolatile memory systems and to methods of operation memory controllers.
A semiconductor memory device is a memory device which is fabricated using semiconductors such as, but not limited to, silicon (Si), germanium (Ge), gallium arsenide (GaAs), indium phosphide (InP), and the like. Semiconductor memory devices are generally classified as either volatile memory devices or nonvolatile memory devices.
The volatile memory devices lose contents stored therein at power-off, and examples thereof static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), and so on. In contrast, nonvolatile memory devices retain stored contents even at power-off, and examples thereof includes read only memory (ROM), a programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable and programmable ROM (EEPROM), flash memory, phase-change RAM (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM), ferroelectric RAM (FRAM), and so on.
Among nonvolatile memories, flash memory in particular has been adopted in a wide variety of applications due to its inherent operational advantages such as high memory capacity, low noise characteristics, low power consumption and fast operating speeds. For example, flash memory is commonly used as a storage medium in mobile systems, such as smart phone, table PC's, and so on.
Flash memory contains semiconductor elements such as a floating gate memory cell, a charge trap flash (CTF) memory cell, and so on. Among these, the CTF memory cell may trap charge in a charge storage layer to store data. In other words, a threshold voltage of the CTF memory cell may be set by varying an amount of charges trapped in a storage layer, and data may be stored according to the set threshold. However, over time the charges trapped in the storage area may leak into a channel layer, which can cause the threshold voltage to drift from an initial state. As a result, data stored in the CTF memory cell may be lost.